The 43 Mauser case is about 60mm long. The Veetterli is about 41mm. Could it be that your 11mm bore is 38mm to the base of the neck? Is this straight walled or does the chamber have a shoulder?

I have seen a couple of late 19th century combination guns with American chamberings like 44/40 Long, etc. The 12 ga makes me think this was possibly made for the American market.